Magnetic bearings support a shaft by magnetic levitation, generally without physical contact, thereby eliminating mechanical wear and exhibiting low friction. One disadvantage to magnetic bearings, however, is that magnetic bearings can drop the shaft during shutdown or failure. If the shaft bottoms out during such a drop event and contacts the magnetic bearings, the magnetic bearings and/or other components coupled to or disposed proximal the shaft can sustain significant damage. To avoid such damage, magnetic bearing systems employ one or more coast-down bearings, also known as auxiliary, backup, secondary, or catcher bearings or bushings. Coast-down bearings are designed to support the shaft while the shaft is slowing down (i.e., coasting down) in the event the primary magnetic bearing drops the shaft. This prevents the shaft from impacting and damaging the magnetic bearings.
Coast-down bearings generally leave a clearance between the bearing and the shaft. During normal operation, the magnetic bearings support the shaft and hold it within this clearance such that the shaft rarely, if ever, touches the coast-down bearing; thus, the coast-down bearing is typically stationary with respect to the shaft during such normal operation. When the magnetic bearing system fails or shuts down, the shaft is caught and/or supported by the coast-down bearing.
To provide the clearance between the bearing and the shaft, however, the coast-down bearing is generally required to have an inside radius that is larger than the outside radius of the shaft disposed therein. Due to the shaft radius being smaller than the coast-down bearing radius, when the shaft is dropped or otherwise comes into contact with the coast-down bearing, the shaft is received along a narrow contact area, generally along an axial line of the coast-down bearing. This small contact area maximizes contact stresses between the shaft and the coast-down bearing, and also promotes whirl during the coast-down. Such maximized contact stresses greatly diminish the useful life of the coast-down bearing and, accordingly, increase the maintenance requirements of the system.
What is needed is a coast-down bearing that minimizes contact stresses during a magnetic bearing failure, shut down, or other drop event, thereby increasing the useful life of the coast-down bearing.